Curiosity science team member Sanjeev Gupta explains how rounded pebbles spotted by the rover are convincing evidence of an ancient streambed on Mars and William Dietrich explores the relationship between river fans found in California's Death Valley on Earth and similar fans in Gale Crater on Mars.

NASA's Curiosity rover found evidence for an ancient, flowing stream on Mars at a few sites, including the rock outcrop pictured here, which the science team has named "Hottah" after Hottah Lake in Canada's Northwest Territories.

There is earlier evidence for the presence of water on Mars, but this evidence - images of rocks containing ancient streambed gravels - is the first of its kind.

Scientists are studying the images of stones cemented into a layer of conglomerate rock.

The sizes and shapes of stones offer clues to the speed and distance of a long-ago stream's flow.

"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich ...